Recent dramatic innovation on lithographic patterning techniques in the manufacture of semiconductors (semiconductor devices) has made design rules as line widths finer and finer. Initially, i-line (i-ray) and g-line (g-ray) were used in lithographic exposure and gave patterns with broad line widths, and the manufactured semiconductor devices thereby had low capacities. However, recent technological development has allowed the use of KrF excimer laser and further recently has allowed the use of ArF excimer laser to give patterns in accordance with dramatically finer design rules. In addition, developments for finer and finer design rules in patterning have been still actively made. Typically, exposure systems which allow immersion lithography have been developed; and techniques for irradiation with extreme-ultraviolet rays (EUV) having shorter wavelengths among ultraviolet rays have also been developed. With finer and finer design rules in patterning, requirements of line edge roughness (LER) as an index of surface roughness of patterns become exacting. For improving LER, there have been examined techniques typically of designing a polymer to have a uniform copolymerization composition; of reducing a difference in polarity between or among monomers to thereby suppress the cohesion of molecular chains; and of designing the polymer to have a lower molecular weight to reduce respective units to be dissolved in an alkali developer during development. The technique of designing the polymer to have a lower molecular weight is effective from the viewpoint of reducing LER, but such a polymer having a lower molecular weight suffers from problems such that it develops crystallinity to have insufficient film-forming properties or has a lowered strength to cause pattern collapse. To solve these problems, there has been proposed a technique of cross-linking polymer chains through a compound capable of cleaving typically with an acid. This technique reduces LER probably for the following reason. Specifically, when such a cross-linked polymer is used, the polymer in unexposed portions has a high molecular weight and shows a sufficient strength, whereas the polymer in exposed portions is cleaved at its cross-links by the action of an acid generated through exposure, and the resulting polymer is dissolved as small structural units in an alkaline developer during development. Thus, units dropping off from the exposed portions become smaller to thereby reduce LER.
Japanese Unexamined Patent Application Publication (JP-A) No. 2002-296782 proposes a polymer which is a cross-linked polymer having one acetal structure per its cross-linking chain. This compound (polymer) is insufficiently efficiently cleaved into units of low molecular weights, because the compound has only one point (acetal structure), which is cleavable by the action of an acid, per its cross-linking chain.
Japanese Unexamined Patent Application Publication (JP-A) No. 2006-003844 discloses a cross-linked polymer which is cross-linked through a divinyl ether compound with a hydroxyl group bound to a polycyclic alicyclic hydrocarbon group in a side chain of the polymer. The repulsing compound (polymer) shows a poor efficiency in cross-linking reaction, probably because the polymer uses, as cross-linking points through the divinyl ether, hydroxyl groups bound to a multivalent alicyclic hydrocarbon, and this causes steric hindrance. In addition, the polymer does not effectively improve LER, because the polymer uses hydroxyl groups as reaction points with the divinyl ether and thereby does not significantly change in solubility in an alkaline developer between before and after cleavage of cross-links.
Japanese Patent No. 3756270 discloses a polymer whose carboxyl groups in polymer side chains are cross-linked through a divinyl ether compound. To have a sufficiently reduced molecular weight in polymer chains after exposure, this compound (polymer) should contain a large amount of carboxyl groups acting as cross-linking points and thereby shows an excessively high sensitivity with respect to an acid. This reduces the maximum allowable limit in variation of the amount of generated acid, which variation is caused typically by fluctuations in exposure, and the polymer has poor handleability. As is described above, there has been found no material having LER and etching resistance in good balance, and demands have been made to provide a resist polymer by which the above problems are improved.